Cloud-to-Ground Lightning throughout the Lifetime of a Severe Storm System in Oklahoma

Abstract

Cloud-to-ground (CG) lightning data are examined relative to digitized radar data for a storm system that occurred in Oklahoma on 26 May 1985. This system evolved through three stages: 1) two lines of cells, one near the dryline and the other 60 km ahead of it; 2) a supercell storm; and 3) a mesoscale convective system (MCS). The behavior of lightning in each stage was different. Initially no ground flashes were observed in either line until reflectivity increased to $46 dBZ and vertically integrated liquid (VIL) increased to $10 kg m22; then ground flash rates remained ,1.2 min21 for .1 h. Most ground flashes in the line of storms near the dryline were negative (18 2CG, 3 1CG), while most in the leading line were positive (11 1CG, 3 2CG), a pattern of polarity opposite to what usually has been observed. Approximately 3 h after radar detected the first storm, ground flash rates increased to .5 min21 and remained so for 6 h. A mesocyclone formed approximately 30 min after flash rates exceeded 5 min 21, and a few positive ground flashes occurred near it. Ground flash rates increased briefly to .20 min21 as the mesocyclone dissipated and then remained .10 min21 as a squall line formed along the outflow boundary from the dissipating supercell and produced a stratiform region. Most ground flashes in this MCS occurred in the convective line and had negative polarity. The few ground flashes in the stratiform region tended to be positive (42 1CG, 32 2CG during 3 h). Durin g1ho f theMCS, ground flash rates decreased and then increased again simultaneously in both the convective and stratiform regions, a previously undocumented behavior. It is possible that this was caused by updrafts in both the convective line and stratiform region changing at roughly the same time. It is also possible that most ground flashes in the stratiform region originated near the convective line, and so were influenced by the line. Overall trends in ground flash density, flash relative frequency, reflectivity, VIL, and severe hail reports appeared similar as the storm system evolved.